We report a novel illusion of motion. A stationary colored bar lay on an achromatic background whose luminance was modulated from below to above the bar luminance and back again. The stationary bar appeared to "sway" back and forth in sync with the background modulation. The illusory motion was strongest in the near periphery but could also be observed in the fovea. We explored this illusion using rating and matching methods. Observers viewed a cyan bar (4° long x 0.33° wide) on a luminance modulating background (1 Hz) for a time period of 4 s, and rated the motion impression on a 7 point scale (0: no motion, 6: very strong motion). Stimulus was presented in four different locations (right/left/upper/lower field, all 4 deg eccentricity). Observers also reported the direction of the motion by adjusting the angle of a matching line. Bar luminance was varied on different trials from lower to higher than the background luminance range. Bar orientation was 0 (vertical), 45 or 90 (horizontal) deg. Results: 1. the illusory motion was perceived only when the bar luminance was within the background modulation range; 2. the perceived direction of motion was orthogonal to the target bar. For example, a vertical bar always oscillated horizontally. We have two hypotheses: the first is that when the bar changes its polarity, it is sensed by different channels that are not in exact spatial register, and the second is the momentary uncertainty of the bar's position at the moment when it is equiluminant with the background. We will discuss these possibilities.